Establishing a cryptographic tunnel between a first tunnel endpoint and a second tunnel endpoint where a private key used during the tunnel establishment is remotely located from the second tunnel endpoint

ABSTRACT

A responder device receives, from an initiator device, a request to initiate a cryptographic tunnel between the initiator device and the responder device. The responder device does not include a static private key to be used in an asymmetric cryptography algorithm when establishing the tunnel. The responder device transmits a request to a key server that has access to the static private key and receives a response that is based on at least a result of at least one cryptographic operation using the static private key. The responder device receives from the key server, or generates, a transport key(s) for the responder device to use for sending and receiving data on the cryptographic tunnel. The responder device transmits a response to the initiator device that includes information for the initiator device to generate a transport key(s) that it is to use for sending and receiving data on the cryptographic tunnel.

FIELD

Embodiments relate to the field of network communications; and morespecifically, to establishing a cryptographic tunnel between a firsttunnel endpoint and a second tunnel endpoint where a private key usedduring the tunnel establishment is remotely located from the secondtunnel endpoint.

BACKGROUND

There are several different ways to provide a secure network connectionbetween two computing devices. Secure Sockets Layer (SSL) and TransportLayer Security (TLS), which is the successor to SSL, are commonly usedto provide secure network connections. SSL and/or TLS are commonly usedduring web browsing (e.g., using HTTPS), email, and other Internetapplications. SSL and TLS are described in several Request For Comments(RFCs), including RFC 2246 (describing TLS 1.0), RFC 4346 (describingTLS 1.1), RFC 5246 (describing TLS 1.2), and RFC 6101 (describing SSL3.0). An SSL or TLS client and server negotiate a set of parameters toestablish a secure session in a process called a handshake. Thehandshake uses public key cryptography during the handshake where aprivate key is used to perform a private key operation (e.g., signingcryptographic information, decrypting an encrypted premaster secret).Other common protocols for establishing a cryptographic connectioninclude Secure Shell (SSH) and IPsec that also use a private key toperform a private key operation. Other, less common means ofestablishing a secure network connection such as the Noise protocolexist.

SUMMARY

A cryptographic tunnel is established between a first tunnel endpointand a second tunnel endpoint where a private key used during the tunnelestablishment is remotely located from the second tunnel endpoint. Aresponder device receives, from an initiator device, a first request toinitiate a cryptographic tunnel between the initiator device and theresponder device, where establishing the cryptographic tunnel includesuse of an asymmetric cryptography algorithm, and where the responderdevice does not include a static private key to be used in theasymmetric cryptography algorithm. The responder device transmits asecond request to a key server that has access to the static private keyto be used in the asymmetric cryptography algorithm, where the secondrequest is requesting the key server to use the static private key in atleast one cryptographic operation of the asymmetric cryptographyalgorithm. The responder device receives, from the key server, a firstresponse that is based on at least a result of the at least onecryptographic operation of the asymmetric cryptography algorithm usingthe static private key. The responder device receives from the keyserver, or generates, a first set of one or more transport keys for theresponder device to use for sending and receiving data on thecryptographic tunnel. The responder device transmits a second responseto the initiator device that includes information for the initiatordevice to generate a second set of one or more transport keys that it isto use for sending and receiving data on the cryptographic tunnel. Theresponder device receives, from the initiator device, a first encrypteddata packet over the cryptographic tunnel and decrypts the firstencrypted data packet using at least one of the first set of one or moretransport keys to reveal a first data packet. The responder deviceprocesses the first data packet. The responder device encrypts a seconddata packet using at least one of the first set of one or more transportkeys and transmits the encrypted second data packet over thecryptographic tunnel to the initiator device. The at least onecryptographic operation of the asymmetric cryptography algorithm usingthe static private key may be a Diffie-Hellman computation that affectsat least the generation of the second set of one or more transport keys.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1 illustrates a sequence diagram that illustrates exemplaryoperations for establishing a cryptographic tunnel between an initiatordevice and a responder device where a private key used during the tunnelestablishment is not locally available to the responder device accordingto an embodiment.

FIG. 2 is a flow diagram that illustrates exemplary operations forestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 3 is a flow diagram that illustrates exemplary operations forestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 4A illustrates a first portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 4B illustrates a second portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 4C illustrates a third portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 5A illustrates a first portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 5B illustrates a second portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 5C illustrates a third portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 6A illustrates a first portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 6B illustrates a second portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 6C illustrates a third portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 7A illustrates a first portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 7B illustrates a second portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 7C illustrates a third portion of an exemplary sequence diagramestablishing a cryptographic tunnel between an initiator device and aresponder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment.

FIG. 8 illustrates a block diagram for an exemplary data processingsystem that may be used in some embodiments.

DESCRIPTION OF EMBODIMENTS

A method and apparatus is described for establishing a cryptographictunnel between a first tunnel endpoint and a second tunnel endpointwhere a private key used during the tunnel establishment is locatedremotely from the device of the second tunnel endpoint. The first tunnelendpoint is located on a device that is referred herein as an initiatordevice. The initiator device initiates the establishment of the tunnel.The second tunnel endpoint is located on a device that is referredherein as a responder device. The initiator device and the responderdevice establish the tunnel through a handshake that uses public keycryptography. In conventional solutions, the responder device has localaccess to the private key that is used when establishing the tunnel.However, in embodiments described herein, the responder device does nothave local access to the private key that is used when establishing thetunnel; instead the private key is located remotely on a device that isreferred herein as a key server.

In an embodiment, upon a point during the handshake between theinitiator device and the responder device, the responder device requeststhe key server to at least access and use the private key to perform oneor more private key operations. The key server may also perform one ormore other operations of the handshake such as generating one or moretransport keys to be used by the responder device for decrypting and/orencrypting transport data received over the tunnel.

FIG. 1 illustrates a sequence diagram that illustrates exemplaryoperations for establishing a cryptographic tunnel between an initiatordevice and a responder device where a private key used during the tunnelestablishment is not locally available to the responder device accordingto an embodiment. FIG. 1 includes the initiator device 110, theresponder device 115, and the key server 120. The initiator device 110is initiating a cryptographic tunnel with the responder device 115. Anasymmetric cryptography algorithm is used for establishing thecryptographic tunnel. The initiator device 110 is a computing device(e.g., desktop, laptop, smartphone, mobile phone, tablet, gaming system,set-top box, server, etc.) that includes a client network application112 (e.g., a web browser, a mobile application, or other application)that is capable of accessing network resources and is capable of actingas an initiator in a tunneling protocol. The use of the term “clientnetwork application” does not require that the initiator device 110 bean end-user client device. The client network application 112 will actas the first tunnel endpoint of the tunnel.

The responder device 115 is a computing device that includes the tunnelapplication 117 that establishes the tunnel with the client networkapplication 112 of the initiator device 110. The responder device 115may be a server device. The tunnel application 117 will act as thesecond tunnel endpoint of the tunnel. The responder device 115 does notstore the static private key of the responder that is used in theasymmetric cryptographic algorithm during the tunnel establishment.Instead, the static private key is stored or available to the key server120 (e.g., the private key 125). However, the responder device 115 doeshave access to the static public key that corresponds to the staticprivate key. The key server 120 is a computing device that includes theprivate key 125 and performs at least a private key operation forestablishing the tunnel between the initiator device 110 and theresponder device 115.

The initiator device 110 and the responder device 115 may performpre-messaging computations before starting the handshake forestablishing the tunnel. For instance, the tunnel application 117 maytransmit a public key (that corresponds with the private key 125) to theclient network application 112. Alternatively, the client networkapplication 112 may obtain the public key through a public keyrepository or through other ways. The tunnel application 117 and theclient network application 112 may compute one or more values that areused during the handshake. For instance, they may each compute aninitial chaining value and initial hash values. The client networkapplication 112 and the tunnel application 117 may each generate anephemeral key pair (an ephemeral public key and ephemeral private key)for the handshake. The ephemeral keys may be a Curve25519 private andpublic keys. The ephemeral keys may be used in a series ofDiffie-Hellman computations.

At operation 1, the client network application 112 of the initiatordevice 110 transmits a request to initiate a cryptographic tunnel to theresponder device 115 that is received at the tunnel application 117. Ina specific example, the request may be a handshake initiation message ifthe WireGuard® protocol is used. The request may include one or morevalues used during the connection establishment (e.g., one or more of asender index, an ephemeral public key of the initiator, an encryptedstatic public key of the initiator, an encrypted timestamp, a firstmessage authentication code (MAC), and a second MAC).

The tunnel application 117 of the responder device 115 receives therequest to initiate the cryptographic tunnel. As described above, inconventional solutions the responder device has local access to a staticprivate key that is used when establishing the tunnel. However, inembodiments described herein, the responder device 115 does not havelocal access to the static private key that is used when establishingthe tunnel. Instead, the private key 125 is located at the key server120. At operation 2, the tunnel application 117 performs at least oneoperation for establishing the cryptographic tunnel. In an embodiment,the tunnel application 117 performs each operation up until the pointwhere the private key is required to perform one or more private keyoperations. In another embodiment, the tunnel application 117 performsless than each operation up until the point where the private key isrequired to perform one or more private key operations.

At operation 3, the responder device 115 transmits a request to the keyserver 120, the request for performing at least one cryptographicoperation using a static private key (e.g., the private key 125). Therequest may also include other information that may be required forprocessing the cryptographic operation. For instance, if thecryptographic operation is a Diffie-Hellman computation that uses theprivate key and other data that is generated for the handshake (e.g., anephemeral public key of the initiator device 110), the request mayinclude the other data for the handshake in which the key server 120cannot independently derive.

The key server 120 receives the request to perform the at least onecryptographic operation. The key server 120 processes the requestincluding performing at least one cryptographic operation. In anembodiment, the key server 120 performs at least two cryptographicoperations using the private key (e.g., two Diffie-Hellman computationsusing the static private key 125). The key server 120 may perform one ormore additional operations to the cryptographic operations includinggenerating the transport keys for the tunnel application 117 to use whenreceiving and sending data over the cryptographic tunnel between theclient network application 112 and the tunnel application 117. Atoperation 5, the key server 120 transmits a response based at least on aresult of the private key operation. The response may include the directvalue of the result of the private key operation and/or may include oneor more values that are derived/computed using the result of the privatekey operation.

The responder device 115 receives the response from the key server 120.The responder device 115 generates a response to the client request toinitiate the cryptographic tunnel at operation 6. The tunnel application117 may perform one or more operations to generate the response. Forinstance, if not generated and received from the key server 120, thetunnel application 117 may generate the transport keys to use whenreceiving and sending data over the cryptographic tunnel between theclient network application 112 and the tunnel application 117. Theresponse may include information for the initiator device 110 togenerate transport keys that it is to use for sending and receiving dataon the cryptographic tunnel. Although not illustrated in FIG. 1, priorto generating the response to the client request, the responder device115 may transmit one or more additional requests to the key server 120to perform one or more additional private key operations, and receiveone or more responses from the key server 120 accordingly. At operation7, the responder device 115 transmits the response to the initiatordevice 110.

The client network application 112 of the initiator device 110 receivesand processes the response including generating the transport keys touse when receiving and sending data over the cryptographic tunnel. Atoperation 8, the client network application 112 transmits an encrypteddata packet over the cryptographic tunnel to the tunnel application 117.The data packet is encrypted with the transport key of the clientnetwork application 112 used for sending data over the cryptographictunnel.

The tunnel application 117 receives the encrypted data packet, decryptsthe encrypted data packet to reveal the data packet, and processes thedata, at operation 9. The tunnel application 117 uses the transport keyfor receiving data to decrypt the encrypted data packet (eithergenerated by the tunnel application 117 or received from the key server120). At operation 10, the tunnel application 117 transmits an encrypteddata packet over the cryptographic tunnel to the client networkapplication 112. The data packet is encrypted with the transport key ofthe tunnel application 117 used for sending data over the cryptographictunnel. The client network application 112 receives, decrypts, andprocesses the encrypted data packet.

Although FIG. 1 shows the client network application 112 transmitting anencrypted data packet to the tunnel application 117 in operation 8 priorto the tunnel application 117 transmitting an encrypted data packet tothe client network application 112 in operation 10, the order of thesepackets may be switched. In other words, the tunnel application 117 maytransmit an encrypted data packet over the established tunnel to theclient network application 112 before the client network application 112transmits an encrypted data packet over the established tunnel to thetunnel application 117.

FIGS. 2 and 3 are flow diagrams that illustrates exemplary operationsfor establishing a cryptographic tunnel between an initiator device anda responder device where the responder device does not have local accessto a static private key used during the tunnel establishment, accordingto an embodiment. The operations of FIGS. 2 and 3 are described withrespect to the exemplary embodiment of FIG. 1. However, the operationsof FIGS. 2 and 3 can be performed by embodiments different than those ofFIG. 1, and the embodiment of FIG. 1 can perform operations differentthan those of FIGS. 2 and 3. The operations of FIG. 2 are described fromthe perspective of the responder device 115. The operations of FIG. 3are described from the perspective of the key server 120.

At operation 210, the responder device 115 receives from the initiatordevice 110, a request to initiate a cryptographic tunnel between theinitiator device 110 and the responder device 115. Establishing thecryptographic tunnel includes use of an asymmetric cryptographyalgorithm that includes use of a static private key on the responderside. The responder device 115 does not include a static private key tobe used in the asymmetric cryptography algorithm (e.g., the private key125). Instead, the static private key is stored or available to the keyserver 120. In an embodiment, the request to initiate the cryptographictunnel may be a handshake initiation message if the WireGuard® protocolis used.

The responder device 115 may perform at least one operation forresponding to the request to initiate the cryptographic tunnel. In anembodiment, the responder device 115 performs each operation up untilthe point where the static private key is needed for performing one ormore private key operations. In another embodiment, the tunnelapplication 117 performs less than each operation up until the pointwhere the private key is required to perform one or more private keyoperations.

At operation 215, the responder device 115 transmits a request to a keyserver 120 that has access to the static private key 125 to be used inthe asymmetric cryptography algorithm. The request in operation 215 isfor the key server 120 to use the static private key 125 in at least onecryptographic operation of the asymmetric cryptography algorithm. The atleast one cryptographic operation may be a Diffie-Hellman computationthat affects at least the generation of the transport keys. The requestmay include information for the at least one cryptographic operation inwhich the key server 120 does not know (e.g., cannot locally derive).For instance, if the cryptographic operation is a Diffie-Hellmancomputation that uses the private key and other data that is generatedfor the handshake (e.g., an ephemeral public key of the initiator device110), the request may include the other data for the handshake in whichthe key server 120 cannot independently derive.

Referring to FIG. 3, at operation 310, the key server 120 receives, fromthe responder device 115, the request to perform at least onecryptographic operation using a static private key. Next, at operation315, the key server 120 performs at least one cryptographic operationusing the static private key. In embodiment, the key server 120 performsat least two cryptographic operations using the private key (e.g., twoDiffie-Hellman computations using the static private key 125). The keyserver 120 may perform one or more additional operations to thecryptographic operations including generating the transport keys for thetunnel application 117 to use when receiving and sending data over thecryptographic tunnel between the client network application 112 and thetunnel application 117.

Next, at operation 320, the key server 120 transmits a response to theresponder device 115 that is based at least on a result of the at leastone cryptographic operation. The response may include the direct valueof the result of the private key operation and/or may include one ormore values that are derived/computed using the result of the privatekey operation. In an embodiment, the response may include the transportkeys for the responder device to use when receiving and sending dataover the cryptographic tunnel between the client network application andthe tunnel application.

Returning back to FIG. 2, at operation 220, the responder device 115receives, from the key server 120, a response that is based on at leasta result of the at least one cryptographic operation of the asymmetriccryptography algorithm using the static private key 125 (e.g., theresponse that was transmitted in operation 320). Next, at operation 225,the responder device 115 receives from the key server 120, or generates,a first set of one or more transport keys for the responder device 115to use for sending and receiving data on the cryptographic tunnel. Forexample, the transport keys may include a key for encrypting data to besent to the initiator device 110 and a key for decrypting data receivedfrom the initiator device 110.

The responder device 115 generates a response to the client request. Theresponse includes information for the initiator device to generate asecond set of one or more transport keys that it is to use for sendingand receiving data on the cryptographic tunnel. At operation 230, theresponder device 115 transmits the response to the initiator device 110.In an embodiment, the response may be a handshake response message ifthe WireGuard® protocol is used.

The initiator device 110 receives and processes the response includinggenerating the second set of one or more transport keys for sending andreceiving data on the cryptographic tunnel (e.g., a key for encryptingdata to be sent to the responder device 115 and a key for decryptingdata received from the responder device 115. At operation 235, theresponder device 115 receives, from the initiator device 110, a firstencrypted data packet over the cryptographic tunnel. The first encrypteddata packet is encrypted with one of the second set of transport keys(e.g., the key to encrypt data to be sent to the responder device 115).Next, at operation 240, the responder device 115 decrypts the firstencrypted data packet using at least one of the first set of one or moretransport keys to reveal a first data packet. For example, the responderdevice 115 uses a key for decrypting data received from the initiatordevice 110 to decrypt the first encrypted data packet.

Next, at operation 245, the responder device 115 processes the firstdata packet. Processing the first data packet may take on many differentforms depending on the type of packet. Assuming that the responderdevice 115 determines to reply to the initiator device 110 with a seconddata packet, at operation 250, the responder device 115 encrypts thesecond data packet using at least one of the first set of transportkey(s). For example, the responder device 115 uses a key for encryptingdata to be sent to the initiator device 110 to encrypt the second datapacket. Next, at operation 255, the responder device 115 transmits theencrypted second data packet over the cryptographic tunnel to theinitiator device 110.

Although FIG. 2 shows a particular order of operations performed in acertain embodiment, the order is exemplary and operations can beperformed in a different order. For example, although FIG. 2 describesthe responder device 115 receiving an encrypted data packet from theinitiator device 110 over the cryptographic tunnel prior to theresponder device 115 transmitting an encrypted data packet to theinitiator device 110 over the cryptographic tunnel, the order of theseoperations can be switched. In other words, the responder device 115 maytransmit an encrypted data packet over the cryptographic tunnel to theinitiator device 110 before receiving an encrypted data packet over thecryptographic tunnel from the initiator device 110.

FIGS. 4A-4C illustrate an exemplary sequence diagram establishing acryptographic tunnel between an initiator device and a responder devicewhere the responder device does not have local access to a staticprivate key used during the tunnel establishment, according to anembodiment. The operations of FIGS. 4A-4C are directed to the specifictunneling protocol WireGuard®. Similar operations may be performed for adifferent tunneling protocol.

The client network application 112 generates a handshake initiationmessage 410 according to the WireGuard® protocol. In an embodiment, thishandshake initiation message 410 is unchanged from the WireGuard®protocol. At operation 4.1, the client network application 112 computesan initial handshake hash (h0) as a hash of a label (e.g., the protocolname). At operation 4.2, the client network application 112 sets theinitial chaining key (ck0) as the initial handshake hash (h0). Atoperation 4.3, the client network application 112 computes a firsthandshake hash (h1) as the hash of the initial handshake hash (h0)concatenated with a second label (label 2). At operation 4.4, the clientnetwork application 112 computes a second handshake hash (h2) as thehash of the first handshake hash (h1) concatenated with the staticpublic key of the tunnel application 117 (S.Pub.R) (the public key thatcorresponds with the private key 125). At operation 4.5, the clientnetwork application 112 generates an ephemeral private key (E.Priv.I)and an ephemeral public key (E.Pub.I). The ephemeral keys may be aCurve25519 private and public keys. The ephemeral keys may be used in aseries of Diffie-Hellman computations. At operation 4.6, the clientnetwork application 112 computes a third handshake hash (h3) as the hashof the second handshake hash (h2) concatenated with the ephemeral publickey of the client network application 112 (E.Pub.I). At operation 4.7,the client network application 112 computes a first chaining key (ck1)as the result of a 1-tuple HMAC-based Extract-and-Expand Key DerivationFunction (HKDF) taking as input the initial chaining key (ck0) and theephemeral public key of the client network application 112 (E.Pub.I). Atoperation 4.8, the client network application 112 computes a firstshared secret (es) as the result of a Diffie-Hellman operation taking asinput the ephemeral private key of the client network application 112(E.Priv.I) and the static public key of the tunnel application 117(S.Pub.R). At operation 4.9, the client network application 112 computesa second chaining key (ck2) and an initial handshake cipher key (k0) asthe result of a 2-tuple HKDF taking as input the first chaining key(ck1) and the first shared secret (es). At operation 4.10, the clientnetwork application 112 encrypts the static public key of the clientnetwork application 112 using an authenticated encryption withassociated data (AEAD) encryption algorithm, taking as input the initialhandshake cipher key (k0), 0, the static public key of the clientnetwork application 112 (S.Pub.I), and the third handshake hash (h3). Atoperation 4.11, the client network application 112 computes a fourthhandshake hash (h4) as the hash of the third handshake hash (h3)concatenated with the encrypted static public key of the client networkapplication 112 (Enc.S.Pub.I). At operation 4.12, the client networkapplication 112 computes a second shared secret (ss) as the result of aDiffie-Hellman operation taking as input the static private key of theclient network application 112 (S.Priv.I) and the static public key ofthe client network application 112 (S.Pub.I). At operation 4.13, theclient network application 112 computes a third chaining key (ck3) and afirst handshake cipher key (k1) as the result of a 2-tuple HKDF takingas input the second chaining key (ck2) and the second shared secret(ss). At operation 4.14, the client network application 112 encrypts atimestamp as using an AEAD encryption algorithm taking as input thefirst handshake cipher key (k1), 0, the timestamp, and the fourthhandshake hash (h4). At operation 4.15, the client network application112 computes a fifth handshake hash (h5) as the hash of the fourthhandshake hash (h4) concatenated with the encrypted-timestamp(enc-time). At operation 4.16, the client network application 112computes a first message authentication code (MAC1) as a MAC of the hashvalue of the concatenation of a MAC label (Label-MAC1) and the staticpublic key of the tunnel application 117, and all the bytes of themessage prior to the MAC1 field (pkt.off.1) (including the data startingat the Type field and ending before the MAC1 field). At operation 4.17,the client network application 112 computes a second MAC (MAC2) as 0 orthe MAC of the most recent cookie, if any, (within 120 seconds) receivedfrom the tunnel application 117 and all the bytes of the message priorto the MAC2 field (pkt.off.2). At operation 4.18, the client networkapplication 4.18 generates and transmits the handshake initiationmessage 410. The handshake initiation message 410 includes the ephemeralpublic key of the client network application 112 (E.Pub.I), theencrypted static public key of the client network application 112(Enc.S.Pub.I), the encrypted timestamp (enc-time), the first MAC (MAC1)and the second MAC (MAC2).

The tunnel application 117 receives the handshake initiation message410. At operation 4.19, the tunnel application 117 computes an initialhandshake hash (h0) as a hash of a label (e.g., the protocol name). Atoperation 4.20, the tunnel application 117 sets the initial chaining key(ck0) as the initial handshake hash (h0). At operation 4.21, the tunnelapplication 117 computes a first handshake hash (h1) as the hash of theinitial handshake hash (h0) concatenated with a second label (label 2).At operation 4.22, the tunnel application 117 computes a secondhandshake hash (h2) as the hash of the first handshake hash (h1)concatenated with the static public key of the responder device 115 (thepublic key that corresponds with the private key 125). The operations4.1-4.4 are the same as the operations 4.19-4.22. The operations4.19-4.22 may be performed prior to the receipt of the handshakeinitiation message 410. At operation 4.23, the tunnel application 117verifies the first MAC value included in the MAC1 field of the handshakeinitiation message 410. Verifying the first MAC value may includecomputing a MAC value as a MAC of the hash value of the concatenation ofa MAC label (Label-MAC1) and the static public key of the tunnelapplication 117, and all the bytes of the message prior to the MAC1field (pkt.off.1) (including the data starting at the Type field andending before the MAC1 field); and comparing that value to the value inthe MAC1 field. If it is not the same, then the tunnel application 117may stop the handshake. If the values are the same, then the handshakecontinues. At operation 4.24, the tunnel application 117 determines ifit is under load, and if so, verifies the second MAC value included inthe MAC2 field of the handshake initiation message 410. If the underload condition is present, the tunnel application 117 may choose to notprocess the handshake message but instead respond with a cookie replypacket. If the responder device 115 is under load, in an embodiment itonly accepts packet that additionally have a second MAC of the priorbytes of the message that utilize the cookie as the MAC key. The exampleof FIG. 4A assumes that the under load condition is not present or theMAC2 value is verified. At operation 4.25, the tunnel application 117computes a third handshake hash (h3) as the hash of the second handshakehash (h2) concatenated with the ephemeral public key of the clientnetwork application 112 (E.Pub.I). At operation 4.26, the tunnelapplication 117 computes a first chaining key (ck1) as the result of a1-tuple HKDF taking as input the initial chaining key (ck0) and theephemeral public key of the client network application 112 (E.Pub.I).

The next operation in the handshake that would conventionally beperformed by the tunnel application 117 using the static private key125. However, the tunnel application 117 does not have local access tothe static private key 125. Accordingly, at operation 4.27, the tunnelapplication 117 transmits a private key operation request 415 to the keyserver 120. The private key operation request 415 includes the ephemeralpublic key of the client network application 112.

Continuing at FIG. 4B, the key server 120 receives and processes theprivate key operation request 415. At operation 4.28, the key server 120computes a first shared secret (es) as the result of a Diffie-Hellmanoperation taking as input the static private key 125 of the tunnelapplication 117 (S.Priv.R) and the ephemeral public key of the clientnetwork application 112. At operation 4.29, the key server 120 transmitsa message 420 that includes the first shared secret (es) to the tunnelapplication 117.

At operation 4.30, the tunnel application 117 computes a second chainingkey (ck2) and an initial handshake cipher key (k0) as the result of a2-tuple HKDF taking as input the first chaining key (ck1) and the firstshared secret (es). At operation 4.31, the tunnel application 117decrypts the encrypted static public key of the client networkapplication 112 using an AEAD decryption algorithm, taking as input theinitial handshake cipher key (k0), 0, the encrypted static public key ofthe client network application 112 (Enc.S.Pub.I), and the thirdhandshake hash (h3) to reveal the static public key of the clientnetwork application 112 (S.Pub.I). At operation 4.32, the tunnelapplication 117 computes a fourth handshake hash (h4) as the hash of thethird handshake hash (h3) concatenated with the encrypted static publickey of the client network application 112 (Enc.S.Pub.I).

The next operation in the handshake that would conventionally beperformed by the tunnel application 117 using the static private key125. However, the tunnel application 117 does not have local access tothe static private key 125. Accordingly, at operation 4.33, the tunnelapplication 117 transmits a private key operation request 425 to the keyserver 120. The private key operation request 425 may include the staticpublic key of the client network application 112. Alternatively, the keyserver 120 accesses the static public key of the client networkapplication 112 through another source such as a public key repository.

The key server 120 receives and processes the private key operationrequest 425. At operation 4.33, the key server 120 computes a secondshared secret (ss) as the result of a Diffie-Hellman operation taking asinput the static private key 125 of the tunnel application 117(S.Priv.R) and the static public key of the client network application112 (S.Pub.I). At operation 4.34, the key server 120 transmits a message430 that includes the second shared secret (ss) to the tunnelapplication 117.

The tunnel application 117 receives the second shared secret (ss) fromthe tunnel application 117 and now may perform the remaining operationsof the tunnel handshake. At operation 4.35, the tunnel application 117computes a third chaining key (ck3) and a first handshake cipher key(k1) as the result of a 2-tuple HKDF taking as input the second chainingkey (ck2) and the second shared secret (ss). At operation 4.36, thetunnel application 117 decrypts the encrypted timestamp included in thehandshake initiation message 410 using an AEAD decryption algorithmtaking as input the first handshake cipher key (k1), 0, the encryptedtimestamp, and the fourth handshake hash (h4). At operation 4.37, thetunnel application 117 computes a fifth handshake hash (h5) as the hashof the fourth handshake hash (h4) concatenated with theencrypted-timestamp (enc-time). At operation 4.38, the tunnelapplication 117 generates an ephemeral private key (E.Priv.R) and anephemeral public key (E.Pub.R). The ephemeral keys may be a Curve25519private and public keys. The ephemeral keys may be used in a series ofDiffie-Hellman computations. At operation 4.39, the tunnel application117 computes a sixth handshake hash (h6) as the hash of the fifthhandshake hash (h5) concatenated with the ephemeral public key of thetunnel application 117 (E.Pub.R). At operation 4.40, the tunnelapplication 117 computes a fourth chaining key (ck4) as the result of a1-tuple HKDF taking as input the third chaining key (ck3) and theephemeral public key of the tunnel application 117 (E.Pub.R). Atoperation 4.41, the tunnel application 117 computes a third sharedsecret (ee) as the result of a Diffie-Hellman operation taking as inputthe ephemeral private key of the tunnel application 117 (E.Priv.R) andthe ephemeral public key of the client network application 112(E.Pub.I). At operation 4.42, the tunnel application 117 computes afifth chaining key (ck5) as the result of a 1-tuple HKDF taking as inputthe fourth chaining key (ck4) and the third shared secret (ee). Atoperation 4.43, the tunnel application 117 computes a fourth sharedsecret (se) as the result of a Diffie-Hellman operation taking as inputthe ephemeral private key of the tunnel application 117 (E.Priv.R) andthe static public key of the client network application 112 (S.Pub.I).At operation 4.44, the tunnel application 117 computes a sixth chainingkey (ck6) as the result of a 1-tuple HKDF taking as input the fifthchaining key (ck5) and the fourth shared secret (se). At operation 4.45,the tunnel application 117 computes a seventh chaining key (ck7), avalue T, and a second handshake cipher key (k2) as the result of a3-tuple HKDF taking as input the sixth chaining key (ck6) and apre-shared symmetric key (Q). At operation 4.46, the tunnel application117 computes a seventh handshake hash (h7) as the hash of the sixthhandshake hash (h6) concatenated with the value T. At operation 4.47,the tunnel application 117 encrypts an empty value using an AEADencryption algorithm taking as input the second handshake cipher key(k2), 0, the empty value, and the seventh handshake hash (h7). Atoperation 4.48, the tunnel application 117 computes an eighth handshakehash (h8) as the hash of the seventh handshake hash (h7) concatenatedwith encrypted empty value (enc-empty). At operation 4.49, the tunnelapplication 117 computes a transport key for decrypting encrypted datareceived from the client network application 112 (T.Recv.R) and atransport key for encrypting data to be sent to the client networkapplication 112 (T.Send.R) using a 2-tuple HKDF taking as input theseventh chaining key (ck7) and an empty string (empty_str). At operation4.50, the tunnel application 117 computes a first MAC (MAC1) as the MACof the hash value of the concatenation of a MAC label (Label-MAC1) andthe static public key of the client network application 112 (S.Pub.I),and all the bytes of the message prior to the MAC1 field (pkt.off.1)(including the data starting at the Type field and ending before theMAC1 field). At operation 4.51, the tunnel application 117 computes asecond MAC (MAC2) as 0 or the MAC of the most recent cookie, if any,(within 120 seconds) received from the client network application 112and all the bytes of the message prior to the MAC2 field (pkt.off.2).

The tunnel application 117 generates and transmits a handshake responsemessage 435 to the client network application 112. The handshakeresponse message 435 includes the ephemeral public key of the tunnelapplication 117 (E.Pub.R), the encrypted empty (Enc-Empty), the firstMAC, and the second MAC.

Continuing at FIG. 4C, the client network application 112 receives andprocesses the handshake response message 435. At operation 4.52, theclient network application 112 verifies the first MAC value included inthe MAC1 field of the handshake response message 435. Verifying thefirst MAC value may include computing a MAC value as a MAC of the hashvalue of the concatenation of a MAC label (Label-MAC1) and the staticpublic key of the client network application 112, and all the bytes ofthe message prior to the MAC1 field (pkt.off.1) (including the datastarting at the Type field and ending before the MAC1 field); andcomparing that value to the value in the MAC1 field. If it is not thesame, then the client network application 112 may stop the handshake. Atoperation 4.53, the client network application 112 determines if anunder load condition exists, and if so, verifies the second MAC valueincluded in the MAC2 field of the handshake response message 435. Theexample of FIG. 4A assumes that the under load condition is not presentor the MAC2 value is verified. At operation 4.54, the client networkapplication 112 computes a sixth handshake hash (h6) as the hash of thefifth handshake hash (h5) concatenated with the ephemeral public key ofthe tunnel application 117 (E.Pub.R). At operation 4.55, the clientnetwork application 112 computes a fourth chaining key (ck4) as theresult of a 1-tuple HKDF taking as input the third chaining key (ck3)and the ephemeral public key of the tunnel application 117 (E.Pub.R). Atoperation 4.56, the client network application 112 computes a thirdshared secret (ee) as the result of a Diffie-Hellman operation taking asinput the ephemeral private key of the client network application 112(E.Priv.I) and the ephemeral public key of the tunnel application 117(E.Pub.R). At operation 4.57, the client network application 112computes a fifth chaining key (ck5) as the result of a 1-tuple HKDFtaking as input the fourth chaining key (ck4) and the third sharedsecret (ee). At operation 4.58, the client network application 112computes a fourth shared secret (se) as the result of a Diffie-Hellmanoperation taking as input the static private key of the client networkapplication 112 (S.Priv.I) and the ephemeral public key of the tunnelapplication 117 (E.Pub.R). At operation 4.59, the client networkapplication 112 computes a sixth chaining key (ck6) as the result of a1-tuple HKDF taking as input the fifth chaining key (ck5) and the fourthshared secret (se). At operation 4.60, the client network application112 computes a seventh chaining key (ck7), a value T, and a secondhandshake cipher key (k2) as the result of a 3-tuple HKDF taking asinput the sixth chaining key (ck6) and a pre-shared symmetric key (Q).At operation 4.61, the client network application 112 computes a seventhhandshake hash (h7) as the hash of the sixth handshake hash (h6)concatenated with the value T. At operation 4.62, the client networkapplication 112 decrypts the encrypted empty value using an AEADdecryption algorithm taking as input the second handshake cipher key(k2), 0, the encrypted-empty value, and the seventh handshake hash (h7).At operation 4.63, the client network application 112 computes an eighthhandshake hash (h8) as the hash of the seventh handshake hash (h7)concatenated with encrypted empty value (enc-empty). At operation 4.64,the client network application 112 computes a transport key forencrypting data to be sent to the tunnel application 117 (T.Send.I) anda transport key for decrypting encrypted data received from the tunnelapplication 117 (T.Recv.I) using a 2-tuple HKDF taking as input theseventh chaining key (ck7) and an empty string (empty_str).

At operation 4.65, the client network application 112 encrypts a packetpayload using an AEAD encryption algorithm taking as input the transportkey for encrypting data to be sent to the tunnel application 117(T.Send.I), a counter (ctr), the packet to be encrypted (Pkt), and anempty string (empty_str). At operation 4.66, the client networkapplication transmits the encrypted data packet 440 to the tunnelapplication 117. The encrypted data packet 440 includes the counter andthe encrypted packet.

The tunnel application 117 receives and processes the encrypted datapacket 440. At operation 4.67, the tunnel application 117 decrypts theencrypted packet (Enc-Pkt) included in the data packet 440 using an AEADdecryption algorithm taking as input the transport key for decryptingdata received from the client network application 112 (T.Recv.R), thecounter (ctr), the encrypted packet, and an empty string (empty_str).The responder device 115 processes the decrypted packet. Assuming thatthe responder device 115 determines to reply to the initiator device 110with a second data packet, at operation 4.68, the tunnel application 117encrypts the second data packet (Pkt2) using an AEAD encryptionalgorithm taking as input the transport key for encrypting data to besent to the client network application 112 (T.Send.R), a second countervalue (ctr2), the packet to be encrypted (Pkt2), and an empty string(empty_str). The tunnel application 117 transmits the encrypted datapacket 445 to the client network application 112. The encrypted datapacket 445 includes the second counter and the second encrypted packet.The client network application 112 receives the encrypted data packet445 and can decrypt it using the transport key for decrypting datareceived from the tunnel application 117. Subsequent data messagesbetween the client network application 112 and the tunnel application117 use similar encryption and decryption.

Although FIG. 4C shows the client network application 112 transmittingan encrypted data packet 440 to the tunnel application 117 prior to thetunnel application 117 transmitting an encrypted data packet 445 to theclient network application 112, the order of these packets may beswitched. In other words, the tunnel application 117 may transmit anencrypted data packet over the established tunnel to the client networkapplication 112 before the client network application 112 transmits anencrypted data packet over the established tunnel to the tunnelapplication 117.

FIGS. 5A-5C illustrate an exemplary sequence diagram establishing acryptographic tunnel between an initiator device and a responder devicewhere the responder device does not have local access to a staticprivate key used during the tunnel establishment, according to anembodiment. The operations of FIGS. 5A-5C are directed to the specifictunneling protocol WireGuard®. Similar operations may be performed for adifferent tunneling protocol. The operations of FIGS. 5A-5C are similarto the operations of FIGS. 4A-4C; however the embodiment shown in FIGS.5A-5C has only one round trip to the key server 120.

The client network application 112 generates a handshake initiationmessage 510 according to the WireGuard® protocol. In an embodiment, thishandshake initiation message 510 is unchanged from the WireGuard®protocol. The operations 5.1-5.18 are the same as the operations4.1-4.18 and are not described herein again. The tunnel application 117receives the handshake initiation message 510 and performs a number ofoperations to process the message including the operations 5.19-5.26,which are the same as the operations 4.19-4.26 and are not describedherein again. The next operation in the handshake would conventionallybe performed by the tunnel application 117 using the static private key125. However, the tunnel application 117 does not have local access tothe static private key 125. Accordingly, at operation 5.27, the tunnelapplication 117 transmits a private key operation request 515 to the keyserver 120. The private key operation request 515 includes the ephemeralpublic key of the client network application 112 (E.Pub.I), theencrypted static public key of the client network application 112(Enc.S.Pub.I), the first chaining key (ck1), and the third handshakehash (h3).

Continuing at FIG. 5B, the key server 120 receives and processes theprivate key operation request 515. At operation 5.28, the key server 120computes a first shared secret (es) as the result of a Diffie-Hellmanoperation taking as input the static private key 125 of the tunnelapplication 117 (S.Priv.R) and the ephemeral public key of the clientnetwork application 112 (E.Pub.I). At operation 5.29, the key server 120computes a second chaining key (ck2) and an initial handshake cipher key(k0) as the result of a 2-tuple HKDF taking as input the first chainingkey (ck1) and the first shared secret (es). At operation 5.30, the keyserver 120 decrypts the encrypted static public key of the clientnetwork application 112 using an AEAD decryption algorithm, taking asinput the initial handshake cipher key (k0), 0, the encrypted staticpublic key of the client network application 112 (Enc.S.Pub.I), and thethird handshake hash (h3) to reveal the static public key of the clientnetwork application 112 (S.Pub.I). At operation 5.31, the key server 120computes a fourth handshake hash (h4) as the hash of the third handshakehash (h3) concatenated with the encrypted static public key of theclient network application 112 (Enc.S.Pub.I). At operation 5.32, the keyserver 120 computes a second shared secret (ss) as the result of aDiffie-Hellman operation taking as input the static private key 125 ofthe tunnel application 117 (S.Priv.R) and the static public key of theclient network application 112 (S.Pub.I). At operation 5.33, the keyserver 120 computes a third chaining key (ck3) and a first handshakecipher key (k1) as the result of a 2-tuple HKDF taking as input thesecond chaining key (ck2) and the second shared secret (ss).

The key server 120 transmits a response 520 to the tunnel application117 that includes the static public key of the client networkapplication 112 (S.Pub.I), the fourth handshake hash (h4), the thirdchaining key (ck3), and the first handshake cipher key (k1). The tunnelapplication 117 receives the response 520 from the key server 120 andnow may perform the remaining operations of the tunnel handshakebeginning at operation 5.34. The remaining operations (5.34-5.49 are thesame as the operations 4.36-4.51 and will not be described again herein.The tunnel application 117 transmits a handshake response message 525 tothe client network application 112. The handshake response message 525takes the same form as the handshake response message 435 and includesthe ephemeral public key of the tunnel application 117 (E.Pub.R), theencrypted empty (Enc-Empty), the first MAC, and the second MAC.Referring to FIG. 5C, the operations 5.52-5.66 performed by the clientnetwork application 112 are the same as the operations 4.52-4.66 and arenot described herein again. The initial data packet 530 takes the sameformat as the initial data packet 440 and will not be described again.The tunnel application performs the operations 5.67 and 5.68, which arethe same as the operations 4.67 and 4.68. Also, the data packet 535takes the same format as the data packet 445.

Although FIG. 5C shows the client network application 112 transmittingan encrypted data packet 530 to the tunnel application 117 prior to thetunnel application 117 transmitting an encrypted data packet 535 to theclient network application 112, the order of these packets may beswitched. In other words, the tunnel application 117 may transmit anencrypted data packet over the established tunnel to the client networkapplication 112 before the client network application 112 transmits anencrypted data packet over the established tunnel to the tunnelapplication 117.

FIGS. 6A-6C illustrate an exemplary sequence diagram establishing acryptographic tunnel between an initiator device and a responder devicewhere the responder device does not have local access to a staticprivate key used during the tunnel establishment, according to anembodiment. The operations of FIGS. 6A-6C are directed to the specifictunneling protocol WireGuard®. Similar operations may be performed for adifferent tunneling protocol. The operations of FIGS. 6A-6C are similarto the operations of FIGS. 5A-5C; however the embodiment shown in FIGS.6A-6C has the key server 120 computing the transport keys on behalf ofthe tunnel application 117.

The client network application 112 generates a handshake initiationmessage 610 according to the WireGuard® protocol. In an embodiment, thishandshake initiation message 610 is unchanged from the WireGuard®protocol. The operations 6.1-6.18 are the same as the operations4.1-4.18 and are not described herein again. The tunnel application 117receives the handshake initiation message 610 and performs a number ofoperations to process the message including the operations 6.19-6.26,which are the same as the operations 4.19-4.26 and are not describedherein again. The next operation in the handshake would conventionallybe performed by the tunnel application 117 using the static private key125. However, the tunnel application 117 does not have local access tothe static private key 125. Accordingly, at operation 6.27, the tunnelapplication 117 transmits a private key operation request 615 to the keyserver 120. The private key operation request 615 includes the ephemeralpublic key of the client network application 112 (E.Pub.I), theencrypted static public key of the client network application 112(Enc.S.Pub.I), the encoded timestamp (enc-time), the first chaining key(ck1) and the third handshake hash (h3).

Continuing at FIG. 6B, the key server 120 receives and processes theprivate key operation request 615. At operation 6.28, the key server 120computes a first shared secret (es) as the result of a Diffie-Hellmanoperation taking as input the static private key 125 of the tunnelapplication 117 (S.Priv.R) and the ephemeral public key of the clientnetwork application 112 (E.Pub.I). At operation 6.29, the key server 120computes a second chaining key (ck2) and an initial handshake cipher key(k0) as the result of a 2-tuple HKDF taking as input the first chainingkey (ck1) and the first shared secret (es). At operation 6.30, the keyserver 120 decrypts the encrypted static public key of the clientnetwork application 112 using an AEAD decryption algorithm, taking asinput the initial handshake cipher key (k0), 0, the encrypted staticpublic key of the client network application 112 (Enc.S.Pub.I), and thethird handshake hash (h3) to reveal the static public key of the clientnetwork application 112 (S.Pub.I). At operation 6.31, the key server 120computes a fourth handshake hash (h4) as the hash of the third handshakehash (h3) concatenated with the encrypted static public key of theclient network application 112 (Enc.S.Pub.I). At operation 6.32, the keyserver 120 computes a second shared secret (ss) as the result of aDiffie-Hellman operation taking as input the static private key 125 ofthe tunnel application 117 (S.Priv.R) and the static public key of theclient network application 112 (S.Pub.I). At operation 6.33, the keyserver 120 computes a third chaining key (ck3) and a first handshakecipher key (k1) as the result of a 2-tuple HKDF taking as input thesecond chaining key (ck2) and the second shared secret (ss). Atoperation 6.34, the key server 120 decrypts the encrypted timestampincluded in the handshake initiation message 610 using an AEADdecryption algorithm taking as input the first handshake cipher key(k1), 0, the encrypted timestamp, and the fourth handshake hash (h4). Atoperation 6.35, the key server 120 computes a fifth handshake hash (h5)as the hash of the fourth handshake hash (h4) concatenated with theencrypted-timestamp (enc-time). At operation 6.36, the key server 120generates an ephemeral private key (E.Priv.R) and an ephemeral publickey (E.Pub.R). The ephemeral keys may be a Curve25519 private and publickeys. The ephemeral keys may be used in a series of Diffie-Hellmancomputations. At operation 6.37, the key server 120 computes a sixthhandshake hash (h6) as the hash of the fifth handshake hash (h5)concatenated with the ephemeral public key of the tunnel application 117(E.Pub.R). At operation 6.38, the key server 120 computes a fourthchaining key (ck4) as the result of a 1-tuple HKDF taking as input thethird chaining key (ck3) and the ephemeral public key of the tunnelapplication 117 (E.Pub.R). At operation 6.39, the key server 120computes a third shared secret (ee) as the result of a Diffie-Hellmanoperation taking as input the ephemeral private key of the tunnelapplication 117 (E.Priv.R) and the ephemeral public key of the clientnetwork application 112 (E.Pub.I). At operation 6.40, the key server 120computes a fifth chaining key (ck5) as the result of a 1-tuple HKDFtaking as input the fourth chaining key (ck4) and the third sharedsecret (ee). At operation 6.41, the key server 120 computes a fourthshared secret (se) as the result of a Diffie-Hellman operation taking asinput the ephemeral private key of the tunnel application 117 (E.Priv.R)and the static public key of the client network application 112(S.Pub.I). At operation 6.42, the key server 120 computes a sixthchaining key (ck6) as the result of a 1-tuple HKDF taking as input thefifth chaining key (ck5) and the fourth shared secret (se). At operation6.43, the key server 120 computes a seventh chaining key (ck7), a valueT, and a second handshake cipher key (k2) as the result of a 3-tupleHKDF taking as input the sixth chaining key (ck6) and a pre-sharedsymmetric key (Q). At operation 6.44, the key server 120 computes aseventh handshake hash (h7) as the hash of the sixth handshake hash (h6)concatenated with the value T. At operation 6.45, the key server 120encrypts an empty value using an AEAD encryption algorithm taking asinput the second handshake cipher key (k2), 0, the empty value, and theseventh handshake hash (h7). At operation 6.46, the key server 120computes an eighth handshake hash (h8) as the hash of the seventhhandshake hash (h7) concatenated with encrypted empty value (enc-empty).At operation 6.47, the key server 120 computes a transport key fordecrypting encrypted data received from the client network application112 (T.Recv.R) and a transport key for encrypting data to be sent to theclient network application 112 (T.Send.R) using a 2-tuple HKDF taking asinput the seventh chaining key (ck7) and an empty string (empty_str).

The key server 120 transmits a response 620 to the tunnel application117 that includes the static public key of the client networkapplication 112 (S.Pub.I), the encrypted-empty value (Enc-Empty), andthe transport keys. The tunnel application 117 receives the response 620from the key server 120 and may perform the operations 6.48 and 6.49 togenerate the first MAC and second MAC. Alternatively, the key server 120may generate the first MAC and/or the second MAC and send the generatedvalues in the response to the tunnel application 117.

The tunnel application 117 transmits a handshake response message 625 tothe client network application 112. The handshake response message 625takes the same form as the handshake response message 435 and includesthe ephemeral public key of the tunnel application 117 (E.Pub.R), theencrypted empty (Enc-Empty), the first MAC, and the second MAC.Referring to FIG. 6C, the operations 6.52-6.66 performed by the clientnetwork application 112 are the same as the operations 4.52-4.66 and arenot described herein again. The initial data packet 630 takes the sameformat as the initial data packet 440 and will not be described again.The tunnel application performs the operations 6.67 and 6.68, which arethe same as the operations 4.67 and 4.68. Also, the data packet 635takes the same format as the data packet 445.

Although FIG. 6C shows the client network application 112 transmittingan encrypted data packet 630 to the tunnel application 117 prior to thetunnel application 117 transmitting an encrypted data packet 635 to theclient network application 112, the order of these packets may beswitched. In other words, the tunnel application 117 may transmit anencrypted data packet over the established tunnel to the client networkapplication 112 before the client network application 112 transmits anencrypted data packet over the established tunnel to the tunnelapplication 117.

FIGS. 7A-7C illustrate an exemplary sequence diagram establishing acryptographic tunnel between an initiator device and a responder devicewhere the responder device does not have local access to a staticprivate key used during the tunnel establishment, according to anembodiment. The operations of FIGS. 7A-7C are directed to the specifictunneling protocol WireGuard®. Similar operations may be performed for adifferent tunneling protocol. The operations of FIGS. 7A-7C are similarto the operations of FIGS. 6A-6C; however the embodiment shown in FIGS.7A-7C has the key server 120 performing more operations on behalf of thetunnel application 117 than in FIGS. 6A-6C.

The client network application 112 generates a handshake initiationmessage 710 according to the WireGuard® protocol. In an embodiment, thishandshake initiation message 710 is unchanged from the WireGuard®protocol. The operations 7.1-7.18 are the same as the operations4.1-4.18 and are not described herein again. The tunnel application 117receives the handshake initiation message 710. At some time, the keyserver 120 computes an initial handshake hash (h0) as a hash of a label(e.g., the protocol name) at operation 7.19. At operation 7.20, the keyserver 120 sets the initial chaining key (ck0) as the initial handshakehash (h0). At operation 7.21, the key server 120 computes a firsthandshake hash (h1) as the hash of the initial handshake hash (h0)concatenated with a second label (label 2). At operation 7.22, the keyserver 120 computes a second handshake hash (h2) as the hash of thefirst handshake hash (h1) concatenated with the static public key of theresponder device 115 (the public key that corresponds with the privatekey 125). The operations 7.19-7.22 may be performed prior to the receiptof the handshake initiation message 710, or could be performed after thetunnel application 117 transmits a request for a private key operation.

Referring back to the tunnel application 117, it verifies the first MACvalue included in the MAC1 field of the handshake initiation message 710at operation 7.23. Verifying the first MAC value may include computing aMAC value as a MAC of the hash value of the concatenation of a MAC label(Label-MAC1) and the static public key of the tunnel application 117,and all the bytes of the message prior to the MAC1 field (pkt.off.1)(including the data starting at the Type field and ending before theMAC1 field); and comparing that value to the value in the MAC1 field. Ifit is not the same, then the tunnel application 117 may stop thehandshake. If the values are the same, then the handshake continues. Atoperation 7.24, the tunnel application 117 determines if it is underload, and if so, verifies the second MAC value included in the MAC2field of the handshake initiation message 710. If the under loadcondition is present, the tunnel application 117 may choose to notprocess the handshake message but instead respond with a cookie replypacket. If the responder device 115 is under load, in an embodiment itonly accepts packet that additionally have a second MAC of the priorbytes of the message that utilize the cookie as the MAC key. The exampleof FIG. 7A assumes that the under load condition is not present or theMAC2 value is verified. The next operation in the handshake wouldconventionally be performed by the tunnel application 117 using thestatic private key 125. However, the tunnel application 117 does nothave local access to the static private key 125. Accordingly, atoperation 7.25, the tunnel application 117 transmits a private keyoperation request 715 to the key server 120. The private key operationrequest 715 includes the ephemeral public key of the client networkapplication 112 (E.Pub.I), the encrypted static public key of the clientnetwork application 112 (Enc.S.Pub.I), and the encoded timestamp(enc-time). Since in this embodiment the key server 120 computes theinitial chaining key (ck0), the key server 120 can compute the firstchaining key (ck1). Accordingly, the first chaining key (ck1) does notneed to be included in the private key operation request 715. Also,since in this embodiment the key server 120 computes the secondhandshake hash (h2), the key server 120 can compute the third handshakehash (h3). Accordingly, the third handshake hash (h3) does not need tobe included in the private key operation request 715.

Continuing at FIG. 2B, the key server 120 receives and processes theprivate key operation request 715. At operation 7.26, the key server 120computes a third handshake hash (h3) as the hash of the second handshakehash (h2) concatenated with the ephemeral public key of the clientnetwork application 112 (E.Pub.I). At operation 7.27, the key server 120computes a first chaining key (ck1) as the result of a 1-tuple HKDFtaking as input the initial chaining key (ck0) and the ephemeral publickey of the client network application 112 (E.Pub.I). The key server 120performs the operations 7.28-7.47, which are the same as the operations6.28-6.47, and will not be repeated for the sake of brevity.

The key server 120 transmits a response 720 to the tunnel application117 that includes the static public key of the client networkapplication 112 (S.Pub.I), the encrypted-empty value (Enc-Empty), andthe transport keys. The tunnel application 117 receives the response 720from the key server 120 and may perform the operations 7.48 and 7.49 togenerate the first MAC and second MAC. Alternatively, the key server 120may generate the first MAC and/or the second MAC and send the generatedvalues in the response to the tunnel application 117.

The tunnel application 117 transmits a handshake response message 725 tothe client network application 112. The handshake response message 725takes the same form as the handshake response message 435 and includesthe ephemeral public key of the tunnel application 117 (E.Pub.R), theencrypted empty (Enc-Empty), the first MAC, and the second MAC.Referring to FIG. 7C, the operations 7.52-7.66 performed by the clientnetwork application 112 are the same as the operations 4.52-4.66 and arenot described herein again. The initial data packet 730 takes the sameformat as the initial data packet 440 and will not be described again.The tunnel application performs the operations 7.67 and 7.68, which arethe same as the operations 4.67 and 4.68. Also, the data packet 735takes the same format as the data packet 445.

Although FIG. 7C shows the client network application 112 transmittingan encrypted data packet 730 to the tunnel application 117 prior to thetunnel application 117 transmitting an encrypted data packet 735 to theclient network application 112, the order of these packets may beswitched. In other words, the tunnel application 117 may transmit anencrypted data packet over the established tunnel to the client networkapplication 112 before the client network application 112 transmits anencrypted data packet over the established tunnel to the tunnelapplication 117.

FIG. 8 illustrates a block diagram for an exemplary data processingsystem 800 that may be used in some embodiments. One or more such dataprocessing systems 800 may be used to implement the embodiments andoperations described with respect to the compute servers or otherelectronic devices. The data processing system 800 is an electronicdevice that stores and transmits (internally and/or with otherelectronic devices over a network) code (which is composed of softwareinstructions and which is sometimes referred to as computer program codeor a computer program) and/or data using machine-readable media (alsocalled computer-readable media), such as machine-readable storage media810 (e.g., magnetic disks, optical disks, read only memory (ROM), flashmemory devices, phase change memory) and machine-readable transmissionmedia (also called a carrier) (e.g., electrical, optical, radio,acoustical or other form of propagated signals—such as carrier waves,infrared signals), which is coupled to the processing system 820 (e.g.,one or more processors and connected system components such as multipleconnected chips). For example, the depicted machine-readable storagemedia 810 may store program code 830 that, when executed by theprocessor(s) 820, causes the data processing system 800 to perform anyof the operations described herein.

The data processing system 800 also includes one or more networkinterfaces 840 (e.g., a wired and/or wireless interfaces) that allowsthe data processing system 800 to transmit data and receive data fromother computing devices, typically across one or more networks (e.g.,Local Area Networks (LANs), the Internet, etc.). The data processingsystem 800 may also include one or more input or output (“I/O”)components 850 such as a mouse, keypad, keyboard, a touch panel or amulti-touch input panel, camera, frame grabber, optical scanner, anaudio input/output subsystem (which may include a microphone and/or aspeaker), other known I/O devices or a combination of such I/O devices.Additional components, not shown, may also be part of the system 800,and, in certain embodiments, fewer components than that shown in One ormore buses may be used to interconnect the various components shown inFIG. 8.

The techniques shown in the figures can be implemented using code anddata stored and executed on one or more electronic devices (e.g., acompute server, a client device, a router, an origin server). Suchelectronic devices store and communicate (internally and/or with otherelectronic devices over a network) code and data using computer-readablemedia, such as non-transitory computer-readable storage media (e.g.,magnetic disks; optical disks; random access memory; read only memory;flash memory devices; phase-change memory) and transitorycomputer-readable communication media (e.g., electrical, optical,acoustical or other form of propagated signals—such as carrier waves,infrared signals, digital signals). In addition, such electronic devicestypically include a set of one or more processors coupled to one or moreother components, such as one or more storage devices (non-transitorymachine-readable storage media), user input/output devices (e.g., akeyboard, a touchscreen, and/or a display), and network connections. Thecoupling of the set of processors and other components is typicallythrough one or more busses and bridges (also termed as bus controllers).Thus, the storage device of a given electronic device typically storescode and/or data for execution on the set of one or more processors ofthat electronic device. Of course, one or more parts of an embodiment ofthe invention may be implemented using different combinations ofsoftware, firmware, and/or hardware.

In the preceding description, numerous specific details are set forth inorder to provide a more thorough understanding of the presentembodiments. It will be appreciated, however, by one skilled in the artthat the invention may be practiced without such specific details. Inother instances, control structures, gate level circuits and fullsoftware instruction sequences have not been shown in detail in ordernot to obscure understanding of the embodiments. Those of ordinary skillin the art, with the included descriptions, will be able to implementappropriate functionality without undue experimentation.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

Bracketed text and blocks with dashed borders (e.g., large dashes, smalldashes, dot-dash, and dots) may be used herein to illustrate optionaloperations that add additional features to embodiments of the invention.However, such notation should not be taken to mean that these are theonly options or optional operations, and/or that blocks with solidborders are not optional in certain embodiments of the invention.

While the flow diagrams in the figures show a particular order ofoperations performed by certain embodiments of the invention, it shouldbe understood that such order is exemplary (e.g., alternativeembodiments may perform the operations in a different order, combinecertain operations, overlap certain operations, etc.).

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, can be practiced with modificationand alteration within the spirit and scope of the appended claims. Thedescription is thus to be regarded as illustrative instead of limiting

What is claimed is:
 1. A method in a responder device, comprising:receiving, from an initiator device, a first request to initiate acryptographic tunnel between the initiator device and the responderdevice, wherein establishing the cryptographic tunnel includes use of anasymmetric cryptography algorithm, wherein the responder device does notinclude a static private key to be used in the asymmetric cryptographyalgorithm, and wherein the first request to initiate the cryptographictunnel includes: an ephemeral public key of the initiator device, anencrypted static public key of the initiator device, and an encryptedtimestamp; transmitting a second request to a key server that has accessto the static private key to be used in the asymmetric cryptographyalgorithm, wherein the second request is requesting the key server touse the static private key in at least one cryptographic operation ofthe asymmetric cryptography algorithm, and wherein the second request toinitiate the cryptographic tunnel includes: the ephemeral public key ofthe initiator device, the encrypted static public key of the initiatordevice, and the encrypted timestamp; receiving, from the key server, afirst response that is based on at least a result of the at least onecryptographic operation of the asymmetric cryptography algorithm usingthe static private key, wherein the first response includes: anencrypted empty value, and a static public key of the initiator device;receiving from the key server or generating a first set of one or moretransport keys for the responder device to use for sending and receivingdata on the cryptographic tunnel; transmitting a second response to theinitiator device that includes information for the initiator device togenerate a second set of one or more transport keys that it is to usefor sending and receiving data on the cryptographic tunnel, wherein thesecond response includes: an ephemeral public key of the responderdevice, and the encrypted empty value; receiving, from the initiatordevice, a first encrypted data packet over the cryptographic tunnel;decrypting the first encrypted data packet using at least one of thefirst set of one or more transport keys to reveal a first data packet;processing the first data packet; encrypting a second data packet usingat least one of the first set of one or more transport keys; andtransmitting the encrypted second data packet over the cryptographictunnel to the initiator device.
 2. The method of claim 1, wherein the atleast one cryptographic operation is a Diffie-Hellman computation thataffects at least the generation of the second set of one or moretransport keys.
 3. The method of claim 1, further comprising: whereinthe first request to initiate the cryptographic tunnel includes aMessage Authentication Code (MAC); and prior to transmitting the secondrequest to the key server, verifying the MAC included in the firstrequest.
 4. A non-transitory machine-readable storage medium thatprovides instructions that, if executed by a processor of a responderdevice, will cause the processor to perform operations comprising:receiving, from an initiator device, a first request to initiate acryptographic tunnel between the initiator device and the responderdevice, wherein establishing the cryptographic tunnel includes use of anasymmetric cryptography algorithm, wherein the responder device does notinclude a static private key to be used in the asymmetric cryptographyalgorithm, and wherein the first request to initiate the cryptographictunnel includes: an ephemeral public key of the initiator device, anencrypted static public key of the initiator device, and an encryptedtimestamp; transmitting a second request to a key server that has accessto the static private key to be used in the asymmetric cryptographyalgorithm, wherein the second request is requesting the key server touse the static private key in at least one cryptographic operation ofthe asymmetric cryptography algorithm, and wherein the second request toinitiate the cryptographic tunnel includes: the ephemeral public key ofthe initiator device, the encrypted static public key of the initiatordevice, and the encrypted timestamp; receiving, from the key server, afirst response that is based on at least a result of the at least onecryptographic operation of the asymmetric cryptography algorithm usingthe static private key, wherein the first response includes: anencrypted empty value, and a static public key of the initiator device;receiving from the key server or generating a first set of one or moretransport keys for the responder device to use for sending and receivingdata on the cryptographic tunnel; transmitting a second response to theinitiator device that includes information for the initiator device togenerate a second set of one or more transport keys that it is to usefor sending and receiving data on the cryptographic tunnel, wherein thesecond response includes: an ephemeral public key of the responderdevice, and the encrypted empty value; receiving, from the initiatordevice, a first encrypted data packet over the cryptographic tunnel;decrypting the first encrypted data packet using at least one of thefirst set of one or more transport keys to reveal a first data packet;processing the first data packet; encrypting a second data packet usingat least one of the first set of one or more transport keys; andtransmitting the encrypted second data packet over the cryptographictunnel to the initiator device.
 5. The non-transitory machine-readablestorage medium of claim 4, wherein the at least one cryptographicoperation is a Diffie-Hellman computation that affects at least thegeneration of the second set of one or more transport keys.
 6. Thenon-transitory machine-readable storage medium of claim 4, wherein theoperations further comprise: wherein the first request to initiate thecryptographic tunnel includes a Message Authentication Code (MAC); andprior to transmitting the second request to the key server, verifyingthe MAC included in the first request.
 7. A responder device,comprising: a processor; and a non-transitory machine-readable storagemedium that provides instructions that, if executed by the processor ofa responder device, will cause the processor to perform operationscomprising: receiving, from an initiator device, a first request toinitiate a cryptographic tunnel between the initiator device and theresponder device, wherein establishing the cryptographic tunnel includesuse of an asymmetric cryptography algorithm, wherein the responderdevice does not include a static private key to be used in theasymmetric cryptography algorithm, and wherein the first request toinitiate the cryptographic tunnel includes: an ephemeral public key ofthe initiator device, an encrypted static public key of the initiatordevice, and an encrypted timestamp, transmitting a second request to akey server that has access to the static private key to be used in theasymmetric cryptography algorithm, wherein the second request isrequesting the key server to use the static private key in at least onecryptographic operation of the asymmetric cryptography algorithm, andwherein the second request to initiate the cryptographic tunnelincludes: the ephemeral public key of the initiator device, theencrypted static public key of the initiator device, and the encryptedtimestamp, receiving, from the key server, a first response that isbased on at least a result of the at least one cryptographic operationof the asymmetric cryptography algorithm using the static private key,wherein the first response includes: an encrypted empty value, and astatic public key of the initiator device, receiving from the key serveror generating a first set of one or more transport keys for theresponder device to use for sending and receiving data on thecryptographic tunnel, transmitting a second response to the initiatordevice that includes information for the initiator device to generate asecond set of one or more transport keys that it is to use for sendingand receiving data on the cryptographic tunnel, wherein the secondresponse includes: an ephemeral public key of the responder device, andthe encrypted empty value, receiving, from the initiator device, a firstencrypted data packet over the cryptographic tunnel, decrypting thefirst encrypted data packet using at least one of the first set of oneor more transport keys to reveal a first data packet, processing thefirst data packet, encrypting a second data packet using at least one ofthe first set of one or more transport keys, and transmitting theencrypted second data packet over the cryptographic tunnel to theinitiator device.
 8. The responder device of claim 7, wherein the atleast one cryptographic operation is a Diffie-Hellman computation thataffects at least the generation of the second set of one or moretransport keys.
 9. The responder device of claim 7, wherein theoperations further comprise: wherein the first request to initiate thecryptographic tunnel includes a Message Authentication Code (MAC); andprior to transmitting the second request to the key server, verifyingthe MAC included in the first request.